1. Field of the Invention
This invention relates to a photovoltaic element for generating photovoltaic power by incident light, and a semiconductor device having hetero junction such as a thin film transistor used in a liquid crystal display or the like and a manufacturing method thereof.
2. Description of the Prior Art
A photovoltaic element is generally categorized into single crystalline-, polycrystalline-, and amorphous-based type depending on a type of a semiconductor of a part for converting absorbed light mainly into electric current. A photovoltaic element including a crystalline substrate of crystalline semiconductor of single crystalline silicon or polycrystalline silicon, and an amorphous silicon semiconductor layer formed on the substrate and forming semiconductor junction between the crystalline substrate and the amorphous silicon semiconductor layer has been studied. For example, U.S. Pat. No. 5,213,628 discloses a photovoltaic element of this kind. In this photovoltaic element, an i-type intrinsic amorphous semiconductor layer of a few to 250 xc3x85 substantially not containing impurity is interposed on a junction interface in forming semiconductor junction by combining a crystalline semiconductor and an amorphous semiconductor having different conductivity from each other so as to improve interface characteristics and photovoltaic conversion characteristics. The i-type amorphous semiconductor layer interposed on the junction interface of the crystalline semiconductor and the amorphous semiconductor is preferred to be a film as thin as possible in order not to reduce light reaching to the crystalline semiconductor as a power generation layer, and it is formed to be not more than 100 xc3x85.
A textured surface of the single crystalline silicon of a crystalline photovoltaic element using single crystalline silicon which has excellent photovoltaic conversion characteristics can reduce reflection and can utilize light effectively by elongating an optical path length in the semiconductor layer by scattering incident light. However, in a photovoltaic element of a structure which an i-type amorphous semiconductor layer is interposed on the junction interface between the crystalline semiconductor and the amorphous semiconductor as disclosed in U.S. Pat. No. 5,213,628, it is difficult to form a thin i-type amorphous semiconductor uniformly on a textured surface of the crystalline semiconductor. It is difficult to form the amorphous semiconductor on a projected part of the crystalline semiconductor by general plasma CVD in forming the very thin i-type amorphous semiconductor layer of a few-100 xc3x85. Therefore, it is difficult to form a uniform thin film on the entire textured surface of the crystalline semiconductor by this method, and a thickness of the i-type amorphous semiconductor layer is required to be thick more than necessary in order to provide a high open circuit voltage, and light reaching to the crystalline semiconductor as a power generating layer reduces.
JP 11-112011, A discloses a method for solving the above problem. In this disclosure, an i-type amorphous semiconductor layer of a predetermined thickness is formed on a one conductive type crystalline semiconductor and an impurity layer is formed on the surface by exposing to plasma containing other conductive type impurity so that an i-type amorphous semiconductor layer is interposed on a junction interface between the crystalline semiconductor and the amorphous semiconductor to obtain high conversion efficiency.
In the above JP 11-112011, A, a reduced open circuit voltage caused by optical loss and uneven film thickness can be prevented, but the interface characteristics between the transparent conductive film formed on the semiconductor layer surface and the semiconductor is degraded because of damage on the surface of the amorphous semiconductor layer generated by plasma exposure on the amorphous semiconductor surface, resulting in degradation of F.F. (fill factor).
This invention was made to solve the existing problems and to provide a highly efficient semiconductor device by eliminating a damage caused by plasma and maintaining interface characteristics between the amorphous semiconductor layer and an electrode.
A semiconductor device of this invention comprises a one conductive type crystalline semiconductor substrate, an amorphous semiconductor layer not containing impurity for reducing electric resistance formed on the crystalline semiconductor substrate, an other conductive type amorphous semiconductor layer doped with other conductive type impurity formed by exposing the crystalline semiconductor substrate having the amorphous semiconductor layer formed thereon in an atmosphere of excited gas containing the other conductive type impurity, and an other conductive type amorphous semiconductor thin film formed on the other conductive type amorphous semiconductor layer by chemical vapor deposition.
A surface of the crystalline substrate is textured.
The semiconductor device of this invention has a structure of the other conductive type amorphous semiconductor layer that the i-type amorphous semiconductor layer not containing impurity, the other conductive type semiconductor layer with impurity diffused on the i-type amorphous semiconductor layer by plasma, and the amorphous semiconductor layer with impurity diffused by plasma are laminated on the one conductive type crystalline substrate. With this structure, even when the crystalline substrate with a textured surface is used, the i-type amorphous semiconductor layer of a predetermined thin film is uniformly formed and a good interface with little damage by plasma on an outermost surface of the amorphous semiconductor layer is formed.
The amorphous semiconductor layer substantially not containing impurity for reducing electric resistance formed on the crystalline semiconductor substrate is hydrogenated amorphous silicon, and the other conductive type amorphous semiconductor layer formed by doping other conductive type impurity to the hydrogenated amorphous silicon may be formed to be a thin film of 10-50 xc3x85 in thickness.
The amorphous semiconductor layer substantially not containing impurity for reducing electric resistance formed on the crystalline semiconductor substrate is hydrogenated amorphous silicon carbide, and the other conductive type amorphous semiconductor layer formed by doping other conductive type impurity to the hydrogenated amorphous silicon carbide may be formed to be a thin film of 20-100 xc3x85 in thickness.
The other conductive type amorphous semiconductor layer formed by CVD may be hydrogenated amorphous silicon carbide.
With this hydrogenated amorphous silicon carbide layer, when the other conductive type amorphous semiconductor thin film is a p-type amorphous semiconductor layer, a band gap can be wide and absorption of light can be reduced.
A manufacturing method of a semiconductor device of this invention comprises a process for forming an amorphous semiconductor layer substantially not containing impurity for reducing electric resistance on a surface of a one conductive type crystalline semiconductor substrate, a process for forming an other conductive type plasma doping layer by diffusing impurity on the amorphous semiconductor layer by exposing the crystalline semiconductor substrate with the amorphous semiconductor layer formed thereon in an atmosphere of excited gas containing other conductive type impurity, a process for forming an amorphous semiconductor thin film layer containing the other conductive type impurity by chemical vapor deposition on the plasma doping layer.
A surface of the crystalline substrate is textured.
The amorphous semiconductor layer substantially not containing the impurity for reducing electric resistance formed on the crystalline semiconductor substrate is hydrogenated amorphous silicon, and the other conductive type plasma doping layer is formed to be a thin film of 10-50 xc3x85.
The amorphous semiconductor layer substantially not containing the impurity for reducing electric resistance on a surface of the crystalline semiconductor substrate is hydrogenated amorphous silicon carbide, and the other conductive type plasma doping layer is a thin film of 20-100 xc3x85.
The process in this invention forms the i-type amorphous semiconductor layer not containing impurity, the other conductive type semiconductor layer with impurity diffused on the i-type amorphous semiconductor layer by plasma, and the conductive layer including the impurity for reducing electric resistance formed on the amorphous semiconductor layer with impurity diffused by plasma on the one conductive type crystalline substrate so as to be a very thin film. With this process, even when the crystalline substrate with a textured surface is used, the i-type amorphous semiconductor layer of a predetermined thin film is uniformly formed with this simple process and a good interface with little damage by plasma on an outermost surface of the amorphous semiconductor layer is formed.
The amorphous semiconductor layer substantially not containing impurity for reducing electric resistance formed on the crystalline semiconductor substrate is hydrogenated amorphous silicon carbide. The other conductive type amorphous semiconductor layer formed by doping other conductive type impurity to the hydrogenated amorphous silicon carbide may be formed to be a thin film of 20-100 xc3x85 in thickness.
The amorphous semiconductor layer substantially not containing impurity for reducing electric resistance formed on the crystalline semiconductor substrate is hydrogenated amorphous silicon. The crystalline semiconductor substrate with the hydrogenated amorphous silicon carbide layer substantially not containing impurity for reducing electric resistance formed thereon is exposed in an atmosphere with excited gas containing the other conductive type impurity and the impurity is diffused on the hydrogenated amorphous silicon carbide layer. Gas containing carbon may be mixed in the gas.
The plasma doping by the above method prevents a band gap from narrowing by impurity introduction.
The other conductive type amorphous semiconductor thin film layer formed by CVD is hydrogenated amorphous silicon carbide.
A reaction chamber for forming the amorphous semiconductor layer substantially not containing impurity for reducing electric resistance and a reaction chamber for forming the amorphous semiconductor thin film layer containing other conductive type impurity are separated, and a process for diffusing the impurity to the amorphous semiconductor layer by exposing the crystalline semiconductor substrate having the amorphous semiconductor layer formed thereon in an atmosphere of excited gas containing the other conductive type of impurity is conducted in the reaction chamber for forming the amorphous semiconductor thin film layer containing the other conductive type impurity.